Yes, the charismatic former president remains extremely popular - 66 percent favorability in the latest CNN poll - and he could knock it out of the arena. Mr. Clinton has the potential, in particular, to reach white, working-class voters, a demographic that challenges Mr. Obama. But the election is still going to boil down to how swing voters feel about Mr. Obama's potential to revive the economy over the next four years. Chances are, Clinton's speech will be all but forgotten by Election Day.

In fact, Clinton's address - to be announced Monday, according to the New York Times - could end up hurting Obama.

"There will be people who say it helps Obama, but they're already voting for him," says Larry Sabato, a political scientist at the University of Virginia,Charlottesville. "And it will just remind people that Obama is no Bill Clinton when it comes to the economy."

Clinton, of course, governed during a rare stretch of peace and prosperity, leaving office with the federal budget in surplus. There's no doubt that he will point out the dire economic straits that Obama inherited from his predecessor, President George W. Bush. But increasingly, voters are tired of the blame game and just want Obama to present a credible plan for the next four years.

When news broke of Clinton's role at the convention, the campaign of Republican candidate Mitt Romney pounced.

"After four years of trillion-dollar deficits and anemic economic growth, it's clear President Obama would love to run on President Clinton's record in office," Romney spokesman Ryan Williams said in a statement. "But no amount of showmanship can paper over the differences between these two presidents. Americans deserve a president willing to run on his own record, not the record he wishes he had."

Clearly, Team Romney thinks Clinton's prime-time role at the convention matters. And for Republicans, the Clinton appearance will provide an unflattering contrast with their own convention, which will have just taken place: The Democrats will present a picture of party unity that the Republicans can't match. The junior ex-President Bush won't have been at the Republican convention in Tampa, Fla.

Clinton's presence also allows the Democrats to move beyond the at-times discordant narrative of the Clinton-Obama relationship, both during the 2008 presidential campaign of Clinton's wife, now-Secretary of State Hillary Rodham Clinton, and more recently, in the former president's comments on Mr. Romney and on policy.

In 2008, Bill Clinton famously dissed Obama's candidacy, calling it a "fairy tale." This past spring, he raised eyebrows by going off-message in praising Romney's record at Bain Capital and suggesting that all of the Bush-era tax cuts should be extended into next year.

But after Clinton's public apology, all is forgiven. Now they're united in the common cause of keeping the Democrats' hold on the Oval Office. Clinton, after all, loves nothing more than a hotly contested campaign.

And speaking of campaigns ... we'll be watching Clinton's remarks for any possible clues on his wife's future. Secretary of State Clinton says she's retiring at the end of this term, and looks forward to putting her feet up. But her husband has hinted he wants her to run again. Polls show her to be the instant front-runner for the Democratic nomination in 2016 if she runs.

Of course, Bill Clinton won't go near the subject in his address in Charlotte, N.C. But watch his eyes during the speech. Maybe he'll be blinking out "Hillary '16" in Morse Code.